Alkenes

• What type of hydrocarbons are alkenes?

  Unsaturated hydrocarbons
• What is the general formula for alkenes?

  $C_nH_{2n}$
• Why are double bonds in alkenes reactive?

  They have high electron density
• What type of molecules attack alkenes during addition reactions?

  Electrophiles
• What is an electrophile?

  An electron pair acceptor
• What happens to bromine water when added to an alkene?

  It decolorizes
• What is a carbocation?

  A positively charged carbon intermediate
• What stabilizes a carbocation?

  Alkyl groups pushing electrons
• What type of carbocation is more stable?

  Tertiary carbocation
• What is the product when alkenes react with hydrogen halides?

  Halogenoalkanes
• What is the role of sulfuric acid in alkene reactions?

  It acts as a catalyst
• What is the mechanism of electrophilic addition reactions in alkenes?

  1. Electrophile approaches double bond.
  2. Electrons from double bond attack electrophile.
  3. Carbocation intermediate forms.
  4. Nucleophile attacks carbocation.
  5. Product is formed.
• What are the types of carbocations and their stability?

  • Primary carbocation: one alkyl group, least stable.
  • Secondary carbocation: two alkyl groups, more stable.
  • Tertiary carbocation: three alkyl groups, most stable.
• What happens to the double bond during addition polymerization?

  • The double bond opens up.
  • Monomers join to form a polymer.
• What intermolecular forces are present in most polyalkenes?

  Van der Waals forces
• How does chain length affect the melting point of polyalkenes?

  Longer chains have higher melting points
• What effect does branching have on the properties of polyalkenes?

  Shorter chains are more flexible
• What is PVC commonly used for?

  Drain pipes and guttering
• How do plasticizers affect polymers?

  They make polymers more flexible
• What is the difference between PVC with and without plasticizers?

  • Without plasticizers: hard and brittle
  • With plasticizers: soft and flexible
• How are alkenes produced?

  During thermal cracking of crude oil
• Why are alkenes more reactive than alkanes?

  Due to high electron density in double bond
• What forces exist between alkene molecules?

  Only van der Waals forces
• How do melting and boiling points of alkenes change with chain length?

  They increase with chain length
• Are alkenes soluble in water?

  No, they are not soluble
• What type of reaction can alkenes undergo due to their unsaturation?

  Addition reactions
• Why does HBr form a polar bond?

  Br is more electronegative than hydrogen
• What happens when water is added to the product of H2SO4 electrophilic addition?

  Hydrolysis - an alcohol is formed and sulfuric acid is reformed
• What defines major and minor products in electrophilic addition?

  Major is most stable; minor is less stable
• What is used to test for a carbon-carbon double bond?

  Bromine solution (bromine water)
• What happens to bromine water when added to an alkene?

  The solution is decolourised
• What is addition polymerization?

  • Formation of long chain molecules
  • Small monomers join together
  • No other products formed
  • Double bond opens to form a carbon backbone
• What are the characteristics of addition polymers?

  • Unreactive due to saturation
  • Usually have non-polar side chains
• How are alkenes formed?

  Thermal cracking of crude oil
• Mechanism for electrophilic addition of ethene with Br2

  1,2-dibromoethane

  A) Nucleophile

  B) Arrow 1

  C) Electrophile

  D) Arrow 2

  E) Arrow 3

  
• Mechanism for electrophilic addition of ethene with HBr

  1-bromoethane

  A) Electrophile

  B) Nucleophile

  C) Arrow 2

  D) Arrow 1

  E) Arrow 3

  
• Mechanism for electrophilic addition of ethene with H2SO4

  C2H6SO4

  A) Arrow 1

  B) Electrophile

  C) Arrow 2

  D) Arrow 3

  E) Electrophile

  
• Define polymers


  Long chain molecules made up of many repeating monomers joined together
• Define monomers


  Short chain molecules that when bonded together form a polymer
• Why are polymers unreactive?

  No polar bonds in chain/Non-polar